1. Field
The present invention relates to semiconductors, and, more particularly, to semiconductor devices in integrated circuits.
2. Description of the Related Art
Microwave transistors are required to operate at ever-increasing frequencies with increased demands on power handling capability. For example, transistors that operate in frequency ranges in the multiple GHz range at voltages well in excess of twenty volts are increasingly needed. Compound semiconductors that combine more than one kind of atom in the semiconductor material from groups III-V or II-VI of the periodic table of elements are the material system of choice for high frequency, high efficiency communication applications. In contrast to compound semiconductors, semiconductors that utilize silicon are more frequently used. However, silicon-based semiconductors are limited in frequency range of operation due to inherent electron transport properties of silicon. A known transistor that has a silicon substrate is the silicon LDMOS (Lateral Double-Diffused Metal Oxide Semiconductor) transistor. While typical LDMOS transistors are able to operate with bias voltages well in excess of twenty volts, such transistors are limited to frequencies no greater than the low GHz range (e.g. up to 3 GHz). As a result, the operating efficiency of known LDMOS transistors is less than desired for many modern communication systems. Therefore, others have focused on extending the operating voltage of compound semiconductor devices.
One proposed compound semiconductor device utilizes a structure known as a step gate transistor. A common feature of a step gate transistor is a gate having two regions at the same potential near the channel. A first region forms a Schottky junction to the channel region. A second region is an extension of a gate extending toward a drain over a dielectric region. Another compound semiconductor device utilizes a field-plate transistor, which has a gate with two regions at arbitrary potentials near the channel.
In both devices, significant depletion of the semiconductor surface between the Schottky junction and the drain occurs. While some known step gate and field plate transistors operate at voltages above twenty volts, such transistors operate at a low power density and low efficiency. Because of such limitations of compound semiconductor devices, LDMOS transistors are typically commercially used more prevalently for microwave applications than compound semiconductor devices. However, it is desirable to use a step gate or field plate transistor and gain the advantages of such a device for high voltage applications. Therefore, a need exists for a step gate or a field plate transistor that can be used for high voltage applications.
The use of the same reference symbols in different drawings indicates similar or identical items. Also, those of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.